Project Summary Premature birth (PTB) is estimated to affect 5?18% pregnancies worldwide. Despite extensive public health efforts, PTL has remained an insidious and intractable cause of infant death and long-term illnesses, suggesting that we do not completely understand PTL pathogenesis. Indeed, the prediction, prevention and treatment of PTL have been very challenging. Intrauterine and systemic infection and/or inflammation are recognized pathophysiologic mechanisms that account for 30?40% of PTL, and ~85% of spontaneous PTB cases at <28 weeks and ~65% of those before 37 weeks showed evidences of intrauterine inflammation. To this end, we have focused on B cells, where our expertise lies, in pregnancy and PTL, because B cells are critical effectors and regulators of innate and adaptive immunity that protects the host against infection, and B cell dysfunction crucially contributes to many autoimmune and inflammatory conditions that predispose women to PTL. Our preliminary studies showed that human choriodeciduas harbored B cells, but B cells in PTL choriodeciduas were functionally altered. B cell-deficient mice were markedly more susceptible than wild-type (WT) mice to PTL after systemic inflammation, but B cells conferred interleukin (IL)-10-independent protection against PTL. B cell deficiency in mice diminished the level of uterine progesterone-induced blocking factor 1 (Pibf1), a progesterone-inducible molecule that carries out many effector functions of progesterone in pregnancy. A lower serum and urine PIBF1 concentration in late pregnancy was strongly associated with an increased risk of PTL. More importantly, therapeutic administration of PIBF1 mitigated PTL and uterine inflammation in B cell-deficient mice, and the protective activity resided in the amino (N) terminal part of PIBF1 (nPIBF1). Interestingly, PIBF1 expression by uterine B cells in late gestation was dependent on the mucosal alarmin IL-33, and PTL patients had diminished expression of the ?-chain of IL-33 receptor, ST2L, on choriodecidual B cells and a lower level of active PIBF1 in late gestation choriodecidua. However, the mechanisms by which PIBF1 protects against PTL and the causes of the intriguing B cell defect in PTL are unknown. As we work to develop PIBF1 into a therapeutic agent to prevent PTL, we want to know the answers to these key questions. Employing computational modelling, biochemical methods and mouse models, studies in Aim 1 will determine the mechanism of PIBF1-mediated protection against PTL. Studies in Aim 2 will use molecular and biochemical approaches to determine the causes of the diminished IL-33R? expression that underlies the functional defects of B cell in PTL patients. Our studies will not only catalyze the preclinical and clinical development of nPIBF1 as a therapeutic agent to prevent or treat PTL, but also offer novel and specific targets to predict and mitigate the B cell defects in this prevalent, devastating and intractable condition. Beside the apparent translational value, the work could reveal novel mechanisms of regulation of type 2 immunity by B cells, tissue danger signals and PIBF1 at the nexus of inflammation, tissue homeostasis and endocrinology.